Themes in Jive 6 are packaged and deployed as Zip Files from Admin Console. They are used to address Front End Look and feel and contain mainly CSS , HTML and SOY files .
Once you upload the Theme Zip , the internal files are deployed on the Server under 'themes' folder.
Updating Themes , is simple enough :-
1) Download the existing theme .
2) Unzip & Extract to a Destination Folder .
3) Make changes to the files which you need to change & SAVE .
4) Zip the file again and Upload it to the Themes via Admin Console
Its always better to Test Your theme on the Test Environment , before deploying it in production.
Theme Changes generally don't require an Application Server Restart .